The present invention relates to the processing of thin films, such as those used in the processing of very small structures such as microelectronic devices, for example.
The industry goal of reducing the size of microelectronic devices places greater demands on photolithography as a technology. As more aggressive solutions are pursued to meet such increased demands, thinner polymer films have to be used as anti-reflective coatings (ARCs) and in photoresist imaging layers. The use of thin polymer films, unfortunately, most often leads to device defects, such as those that occur due to long range van der Waals forces. Due to Van der Waals forces, localized thinning of a polymer film on a substrate occurs when the polymer film has insufficient thickness to overcome a tendency to dewet from the substrate. This leads to dewetting defects, also known as “pinhole” defects. An example of this phenomenon is illustrated in FIG. 1 for a bottom anti-reflective coating (BARC) layer disposed on a substrate of silicon dioxide.
FIG. 1 illustrates a free energy curve 10 for a BARC layer disposed on a substrate of silicon dioxide, and a second curve 12 being the second derivative of the free energy curve 10. The BARC layer becomes unstable and has a tendency to dewet catastrophically at a thickness (50 nm) below which the free energy curve 10 turns sharply lower and heads negative. Such catastrophic dewetting is referred to as spinodal dewetting. The location of the zero in the second curve 12 illustrating the second derivative of free energy indicates a crossover point at about 85 nm between a film that dewets spinodally below that thickness and dewets via nucleation and growth of holes above that thickness.
As further shown in FIG. 1, as the overall film thickness is increased, the free energy of the film passes through a maximum and starts to decrease slowly as the film thickness continues to be increased. In the thickness regime just beyond the thickness at which the film spinodally dewets, the film is metastable and can still dewet via a different mechanism. If at some localized point in the film the thickness falls below the 85 nm thickness of the crossover point, the film becomes locally unstable and dewets by a mechanism known as dewetting via nucleation and growth of holes. Consequently, by examining the curves presented in FIG. 1, a BARC film having a thickness of 80 nm, which is less than the crossover point thickness of 85 nm, is highly unstable, and dewets spinodally, rapidly dewetting to droplets. On the other hand, a BARC film having a thickness of 110 nm, does not dewet spinodally, but can still dewet locally via nucleation and growth of holes. When the thickness of the film is increased, however, the occurrence of defects becomes less likely. For example, a BARC film having a thickness of 200 nm is so far from the crossover point on the free energy diagram that random local fluctuations in film thickness no longer result in local instability of the film.
Heretofore, there has been no known solution to this problem other than to increase the thickness of the film, which runs contrary to the industry goal of reducing device size. In addition, advanced lithography processes call for reductions rather than increases in film thicknesses, especially since a thick BARC film unnecessarily increases the difficulty of etching through the BARC film. Similarly, a thick photoresist imaging layer also increases risk of line pattern collapse and reduces the process window.
Currently, it is common to utilize surface treatments such as hexamethyldisilazane (HMDS) prime, prior to applying a coating such as an ARC or a photoresist. Such treatment promotes adhesion by changing the surface tension, and can also affect wettability of the coating by changing the spreading coefficient. However, even when a coating has a positive spreading coefficient, pinholes can still form when instability is present due to long range van der Waals forces. Therefore, pre-treating a surface with a surface treatment such as an HMDS prime, while affecting the size and shape of dewetting defects, does not prevent them from appearing in the first instance.
Accordingly, it would be desirable to provide a method by which the thickness of a film utilized in semiconductor fabrication can be reduced while precluding defects in the film caused by long range van der Waals forces.